1. Field of the Invention
The present invention relates to a pigment dispersion, an ink-jet ink and a printing ink containing the same.
2. Description the Related Art
In general, in the case of producing printed inks, ink-jet inks and the like, it is difficult to stably disperse a pigment at a high concentration. It is known that the incorporation of the pigment having a high concentration into the ink may cause various problems during a production process and in products themselves. For example, a dispersion containing a pigment which comprises fine particles often exhibits a high viscosity. Therefore, it becomes difficult to take the dispersion out of a dispersing machine or to transport it. Moreover, such a dispersion gels sometimes during storage, with the result that even its use may be difficult. Furthermore, the dispersion sometimes causes state defects such as less gloss and insufficient leveling on the surface of color-developed materials. Additionally, in the case that such a dispersion contains a different kind of pigment, there sometimes appear color separation due to aggregation, uneven coloration of color-developed materials by precipitation or the like, and noticeable decrease of a coloring power.
In order to solve such various problems as mentioned above, methods for stably dispersing the pigment have been proposed. For example, such methods are disclosed in U.S. Pat. Nos. 3,370,971 and 2,965,511, Japanese Patent Publication No. S41-2466 and U.S. Pat. No. 2,855,403. Moreover, there have been proposed some methods for mixing a certain kind of varnish with a pigment derivative as a dispersant including a pigment as a main structure and having a substituent such as an acidic group or a basic group in a side chain thereof. Such methods are disclosed in, for example, Japanese Patent Laid-open Nos. S63-305137, H01-247468, and H03-26767. In addition, there has been suggested a method of adding copper phthalocyaninesulfonic acid and a dispersant containing a polyalkyleneimine at a time when the pigment is dispersed. This technique is disclosed in Japanese Patent Publication No. H06-67463.
However, a satisfactory effect has been not necessarily obtained, and thus a technique which is more excellent in a dispersing effect has been desired.